Thermal Performance of External Renders Applied to Concrete Blockwork
Andrew Sapienza1, Alex Torpiano2 & Vincent Buhagiar3
1
Faculty of Architecture & Civil Engineering, University of Malta, Msida MSD 2080, Malta, Email:
andrewsapienza@keyworld.net
2
Faculty of Architecture & Civil Engineering, University of Malta, Msida MSD 2080, Malta, Email:
alex.torpiano@um.edu.mt
3
Faculty of Architecture & Civil Engineering, University of Malta, Msida MSD 2080, Malta, Email:
vincent.buhagiar@um.edu.mt
1 Introduction
The aim of this study was to investigate and
analyse different external renders, available
locally, and to study how their use may
enhance the overall thermal performance of
local concrete blockwork. This study provides
an insight into how the various types of renders
available improve the U-value of a concrete
block wall. Three main types of external
renders were used as the basis of this study.
Results demonstrate that the ‘glass fibre
additive’ type of render helped to greatly
improve the U-value of a bare concrete block
wall. This enhanced U-value, however, is
achieved using more expensive render systems.
Thus, from this study, the energy conscious
designer can assess how, with the help of
specific external renders, a more energy
efficient building could be achieved, or how an
existing building’s thermal efficiency could be
improved.
There are three aspects of performance that
inform the selection of an external finish,
namely:
ƒ Aesthetic quality (colour & texture);
ƒ Cost effectiveness, as compared to other
types of finishes;
ƒ Resilience to adverse weather conditions,
particularly thermal performance.
This paper investigates the issues relating to
thermal performance, but also highlights the
cost associated with choosing alternative but
similar renders.
insufficient to combat the convective cooling
of such severe winds. Over the last decade,
imported
admixtures
or
pre-prepared
composite renders have been used in
applications to local blockwork walls.
Blockwork walls have recently become the
more
popular
material
for
masonry
construction, exceeding the use of indigenous
globigerina limestone. Hence the application of
an effective render to the otherwise highly
porous blockwork is even more important.
3 Renders Used for Tests
In Malta, the conventional cement-based
render is widely used, since it is still the
cheapest to buy, and since it can be used as a
substrate for other systems, for example, with
another thin silicate or polymer render finish
applied to it. Renders are still considered as
merely surface finishes, and therefore The
cheapest possible product, with the best visual
effect, is sought.
Since there are basic similarities in all the
ranges of products offered by the different
companies, it was felt that it would not be
useful to study similar types of render from
different companies. Italian products were used
for the tests, imported by a local company.
This company was interested in the
opportunity to carry out experimental studies
on their products for the local construction
industry. This Company also test their renders
in Sicily, a climate very similar to that of
Malta.
2 Background
3.1 Render Descriptions
Malta is a typical Mediterranean Island, with
hot dry summers and warm wet winters. It also
has windy conditions, all year round,
particularly along the coast. Rendering mortars
with typical local mix designs often prove
The choice of render system was based
primarily on their widespread use, locally, and
secondly because on the insulating properties
claimed for their products.
Three types of renders were used, namely a
cement-based mortar mix, here called render
P1, a porous dehumidifying, marcoporous mix,
here called type P2, and a smooth finish
polystyrene mix with a glass fibre mesh
reinforcement, here named render P3. (To
conform with trade secrecy, no mix design is
divulged by the company; similarly no brands
or importers’ trade names are included in this
text).
The products used were:
P1 – pre-mixed cement base mortar. This is the
most popular render and, since it is pre-packed,
it ensures speed in carrying out the work, with
guaranteed results and constant quality on site.
This render is usually applied at 6–7mm in
thickness. Also, polymer-based resin additives
may be added to the mix, to achieve different
qualities required, such as waterproofing or
increased elasticity.
Fig2: P2, macroporous mix
P3 – this system consists of a smooth finish
adhesive, polystyrene foam and glass fibre
mesh covered again by the smooth finish
adhesive. A final finish coating is applied over
this system. This thermally-insulated façade
system can offer many advantages, apart from
offering a very fast way of thermally insulating
a building. As a result, the interior face of the
wall does not cool down much, stopping
unwanted condensation, dampness and mould.
Fig1: P1, sand and cement mix
P2 – this render is said to be an ‘advancedtechnology dehumidifier’ because of its
macroporous structure, the render also has low
thermal conductivity, thus improving the
thermal insulation of masonry. This render can
be applied in layers of up to 25mm thickness
for maximum insulation efficiency.
Fig 3: P3, polystyrene and glass fibre structure
4 Experimental Methodology
The intention of this study is to carry out
comparative experiments on various types of
renders applied to concrete block, whilst still
keeping to the guidelines recommended by
the Euro-code as specified in EN ISO
8990:2000 (MSA, 2004), but at the same time
achieving results that would really represent
what actually happens on site of newly built
or renovated buildings.
It was decided (after analysing many
methods) that the best test method in order to
minimise all these losses, and to ensure that
all the heat (Q) supplied, only passed through
the test blocks, was to build small concrete
block ‘cells’. In doing so, heat losses were
minimised and controlled. The whole internal
area (A) of the ‘cell’ was taken into account.
This method is based nonetheless on the test
procedure given in the Euro-code EN ISO
8990:2000 (MSA, 2004).
The success of the experiments depends on
the heat supplied (Q) which flows laterally
through the wall area (A) to the outside (cold
side). Therefore it was very important to
minimize heat loss from any other areas of the
experiment set-up by applying thick
insulation at the top and bottom of the cells. If
heat losses were significant, even through the
insulation itself, then the results could be very
misleading, or completely wrong. Special care
was taken, when designing the test procedure,
to ensure that the different tests could be
carried out in exactly the same way, so that if
some heat losses were unavoidable, this
would be a constant throughout all the
different tests.
Fig 4: Thermocouples fixed into position on ‘hot’ face
Two 10mm diameter holes were dug through
the base insulation, to allow the passage of the
thermocouple wires, as well as the wires
leading to the resistors and light bulb, from
the inside to the outside of the cell. The two
holes were dug as distant from one another as
possible so the higher voltage passing through
the wires leading to the resistors would in no
way interfere with the milli-volts passing
through the thermocouple wires. A higher
voltage passing close to the thermocouple
wires, could affect the voltage induced with
the wire, and therefore a wrong reading may
result or different readings may be read for
the same temperature.
5 Test Procedure
Four cells were built from the concrete blocks
provided. These measured 1150mm x 920 x
520mm. These were built on insulation panels
120mm thick, and elevated from the ground to
ensure a good insulation barrier beneath the
cell. The top of the cell was roofed with
300mm thick insulation mineral fibre
insuilation(Rockwool©) to minimise heat
losses from the top. The internal face of the
cell was left bare and the external face was
applied with the different types of renders. In
this way, since the top and bottom of the cell
were well insulated, heat only flowed laterally
through the concrete brick walls and render to
the outside.
Fig 5: Thermocouples fixed into position on ‘hot’ face
Each thermocouple was clearly labelled
according to its assigned channel to avoid
confusion when moving the sensors from one
cell to another. This also easily helped to
identify which sensor may be defective in case
of abnormal readings.
were not so close. This trend was seen in
every set of results. This may have been due
to the fact that during the first test run, the cell
had higher moisture content, since the
environment in the laboratory was quite
damp, and also the bricks may also have been
slightly damp from the application of the
renders. Therefore the first test was used to
‘stabilise’ the cells for the following tests. In
total eight tests were considered valid, and
were in line with EN ISO 8990:2000 (MSA,
2004).
Fig 6: Thermocouples labelled according to their
respective channels and connected to data logger
5.1 Insulation Sandwich of Cell
Great care was taken to ensure that heat loss
from the inside of the cell was minimal from
the top or bottom of the cell. This was to
ensure that heat could flow laterally through
the concrete block and render. The cell was
built raised off the ground and on an insulation
panel 120mm thick. The insulation panel and
the air beneath the panel provided an insulation
barrier.
A graph was plotted by the computer software
throughout the duration of the test, to help
verify the time at which the steady-state had
been reached. The tests were frequently
inspected, to ensure that all was proceeding
well, and that the readings made sense. Final
readings were taken when at least two
successive readings spread over three hours
after near-stability had been reached, and
agreed within 1% (MSA, 2004).
A summary of results is outlined in table 1:
Table 1: U-values in W/m2K.
Cells A,B,C with respective render applied.
Cell /
Bare
Cell Cell Cell
Render type Cell
A
B
C
Bare Cell
4.78
----- ----- ----P1
----4.63 ----- ----P2
--------- 3.61 ----P3
--------- ----- 2.01
Using bare cell (non-rendered) as reference,
the percentage decrease in U-value due to the
applied renders were 3.14%, 24.58%, 57.95%
for P1, P2 and P3 respectively.
Fig 7: Cell sandwiched between insulation
6 Tests and Results
Fourteen tests were carried out in total, which
comprised of 704 hours of testing time. Three
tests were carried out on each cell, but the
first test of each set was discarded, because
they were presumably too short, and the
results achieved, when compared to the
results of the subsequent two tests carried out,
7 Scope for Further Research
Stemming from this paper, one area that merits
further research is the investigation of their
embodied energy for the given conductivity
they achieve.
8 Conclusions
Render P3, reduced the U-value by almost
58% over bare blockwork. However this
financially costs the most out of the three,
standing at €70/m2 compared to €47/m2 &
€14/m2. This additional cost difference is
deemed a worthy investment, as buildings
would be losing less heat through the fabric in
winter, thus making them more sustainable.
Due to the fact the Malta has to import fossil
fuels, these initial investments are becoming
ever more relevant given today’s ever soaring
price of energy (record high price of oil at
$118/barrel-Apr08).
Rendering may also be considered as a valid
alternative to cavity insulation, particularly if
the embodied energy of the render is compared
to an insulating material to achieve the
equivalent U-value.
References
London. A.G, (1981). Thermal properties of
Slotted blockwork, Concrete.
MSA (2004). EN ISO 8990:2000 - Thermal
insulation – Determination of Steady-state
thermal transmission properties- Calibrated
and guarded hot box. Valletta, Malta: Malta
Standards Authority
Pulis, S. (1992). The Thermal properties of
Concrete Blockwork. Unpublished Bachelor’s
dissertation, University of Malta.